1. Field of the Invention
The present invention relates to a developing device that is configured to supply toner into a developer container containing a developer, a process cartridge having this developing device, and an image forming apparatus that uses either the developing device or the process cartridge.
2. Description of the Related Art
In a recent image forming apparatus such as an electrophotographic copier, facsimile device and printer, it has been requested to provide stability of image quality without causing density irregularities or the like even if an image with a high image-area ratio is printed continuously. Therefore, the functions of the developer carrying means of such image forming apparatus were divided to improve developer stirring performance by changing the arrangement of the developer carrying means from the conventional horizontal arrangement to vertical arrangement so that the developer carrying means on the lower side could recover the developer obtained after development from the developer carrier, and the developer carrying means on the upper side could supply developer to the developer carrier.
However, such change was not sufficient to stir the developer well, thus there was a problem that unevenness and reduction of concentration would occur.
Japanese Unexamined Publication No. H11-167260 discloses a developing device that comprises: a housing that stores two-component developer composed of toner and carrier; development means for magnetically carrying the two-component developer in the housing and applying the toner to electrostatic latent image carrier; a first carrying path that is provided in a position to supply the two-component developer to the development means within the housing, stirs/carries the two-component developer in a first direction along an axial direction of the development means, and has first developer carrying means for supplying the two-component developer to the development means; a second carrying path that is provided parallel to the first carrying path within the housing, recovers, from the development means, the two-component developer obtained after development, and has second developer carrying means for stirring/carrying the recovered developer in the same direction as the first direction; and a third carrying path that is provided parallel to the first carrying path, supplied with excessive developer supplied from the first carrying path to the development means and with the recovered developer from the second carrying path, has third developer carrying means for stirring/carrying the supplied excessive developer and the recovered developer in a second direction opposite to the first direction, and supplies, in a circulating manner, stirred developer obtained by mixing the excessive developer and recovered developer. In this apparatus, developer stirring performance was further improved by adding another developer carrying means and providing a stirring section besides the developer recovering section and developer supply section.
However, the problem was that, if the downstream side of the recovering section were excessively filled with the developer, low-concentration developer would be drawn up again into the developer carrier, causing irregularity of image density.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Unexamined Publication No. H11-133710 and Japanese Unexamined Publication No. 2004-077587.